By Laurence Norman and Inti Landauro 

BRUSSELS--The European Commission said Monday it had opened a probe into whether Luxembourg's tax treatment of French energy giant Engie SA breached European Union state-aid laws, the latest in a series of high-profile investigations looking at past tax deals.

The move sets up a potential clash between the EU's executive arm and one of the largest and most powerful French companies in which the French government has a 32.8% stake.

It comes after U.S. accusations that the EU was targeting mainly American companies in its tax probes. Last month, the commission ordered Ireland to collect some EUR13 billion in unpaid taxes from Apple Inc. ($14.5 billion).

Monday's announcement also came as the EU's competition commissioner Margrethe Vestager started a three-day tour of the U.S., her first trip there since the Aug. 31 Apple ruling.

The commission, the EU's executive arm, said it had concerns that several tax rulings made in September 2008 by the Luxembourg authorities may have given GDF Suez group, now called Engie, an unfair advantage over other companies.

The commission said that several tax rulings between different Engie units appeared to allow the company to reduce its tax bills on profits arising in Luxembourg.

The commission said that several tax rulings granted by Luxembourg to Engie appeared to allow the company to reduce its tax bills on profits arising in the country.

The commission is looking at two zero-interest loans that could be converted into equity which were granted by two Engie units to two others in 2009 and 2011. In those transactions, one side deducted the expenses as interest rates, while the other side deducted the income as dividend.

As a result, the commission said, the company may have paid no tax on a "significant proportion of the profits" generated by the transactions.

In a statement, Ms. Vestager said the rulings "seem to contradict national taxation rules and allow GDF Suez to pay less tax than other companies."

The commission didn't say how much money might be at stake in the case. A spokesman, Ricardo Cardoso, said this would only start to become clear when they receive more information from the company and Luxembourg authorities.

"Engie will fully cooperate with the commission on the investigation," a spokeswoman said. She said the company's units in Luxembourg are not shell companies. Roughly 300 people work for Engie in the country, mainly in energy services. The company has an energy trading desk in the city of Luxembourg.

The spokeswoman declined to provide the amount of taxes Engie has paid in Luxembourg since 2008 and how much profit it has made there.

Finance Minister Michel Sapin hasn't made any specific comment on the Engie case yet, but he has repeatedly said companies, whether private or state-owned, should pay taxes in the country where they make profits, an official at the Ministry said Monday.

In a statement following the announcement of the probe, Luxembourg's finance ministry said the government "considers that no special tax treatment or selective advantage has been awarded to any Engie group company in Luxembourg."

A commission spokesman, Ricardo Cardoso, denied that the announcement of the probe into Engie, in which the French government still holds a 33% stake, was timed to coincide with Ms. Vestager's trip to the U.S.

"We will always apply state aid rules to all companies and this independently of whether they are EU-based or U.S.-based companies or based anywhere else for that matter," he said.

The in-depth investigation into Engie is the first to come out of the information gathered from an EU-wide inquiry into tax rulings in 2014. So far, the commission has received information on some 1,000 tax rulings in 23 EU countries.

That exercise was launched after a huge document leak showed how Luxembourg, whose longtime former prime minister is the current commission President Jean-Claude Juncker, helped international companies funnel profits into the country and pay minimal taxes.

Mr. Juncker has said he wasn't involved in the detail of the tax decisions.

Gabriele Steinhauser contributed to this article.

Write to Laurence Norman at laurence.norman@wsj.com and Inti Landauro at inti.landauro@wsj.com

(END) Dow Jones Newswires

September 19, 2016 11:21 ET (15:21 GMT)